Disc refiners are used in the paper manufacturing industry to prepare the cellulose fibers of a paper pulp into a desired condition prior to delivering the pulp to the papermaking machine. In operation, a disc refiner is generally considered to exert a type of abrasive action upon individual fibers and the pulp mass so that the outermost layers of the individual cigar-shaped fibers are frayed. This facilitates the bonding of the fibers.
The fibers are relatively slender, tube-like structural components made up of a number of concentric layers. Each of these layers (called "lamellae") consists of finer structural components (called "fibrils") which are helically wound and bound to one another to form the cylindrical lamellae. The lamellae are in turn bound to one another, thus forming a composite which, in accord with the laws of mechanics, has distinct bending and torsional rigidity characteristics. A relatively hard outer sheath (called the "primary wall") encases the lamellae. The primary wall is often partially removed during the pulping process. The raw fibers, which are relatively stiff and have relatively low surface area when the primary wall is intact, inhibit bond formation and as a result limit the strength of the paper formed from these fibers.
It is generally accepted that it is the purpose of a pulp stock refiner, which is essentially a milling device, to remove the primary wall and break the bonds between the fibrils of the outer layers to yield a frayed surface, and thereby increase the surface area of the fiber multifold. The operation of disc refiners is also generally considered to cause a rapid, frequent flexure over a brief time period of the individual fibers in the pulp mass. This flexing renders the fibers more flexible. This in turn allows the fibers to conform to each other and produce larger areas of intimate contact.
It is the purpose of a stock refiner to modify the fibers in accordance with the above requirements without significantly reducing the length or individual strength of these fibers. U.S. Pat. No. 3,880,368 to Matthew discloses the benefit of repeatedly and gently refining the pulp to ensure that fibers are not extensively damaged. Matthew points out the impracticability of avoiding all fiber damage, and suggests the reduction of fiber damage through the use of plastic or low-modulus materials for the construction of refiner discs.
Matthew also suggests that gentle refining can be accomplished by the use of many blades per plate and operating at relatively high speeds. However, the use of many blades or bars on the plates reduces the flow area. This can reduce the through-put, decreasing the efficiency and increasing the costs of the refining process.
U.S. Pat. No. 3,305,183 to Morden shows curved bars on a the interior of a frustoconical shell refiner section and suggests such bars could be used on refiners of nonfrustoconical shape. Morden does not disclose how to use curved bars on flat plates to obtain the benefits of more gentle refining and higher throughput.
What is needed is a disc refiner that improves through-put with a refining action that is less damaging to individual paper fibers.